Thin-films have been prepared by a number of deposition methods including evaporation, sputtering, laser ablation, and metallo-organic chemical vapor deposition (MOCVD), see, for example, "Thin Film Processes II", (J. L. Vossen and W. Kern, eds.), Academic Press, New York, 1991; "Handbook of Thin Film Technology" (L. I. Maissel and R. Glang, eds.) McGraw Hill, New York, 1970, and R. W. Simon, Solid State Technology, p. 141, September, 1989.
The thin film deposition processes of the prior art typically depend on the substrate temperature. When the temperature of the substrate onto which a thin film such as YBa.sub.2 Cu.sub.3 O.sub.7-x is being deposited is low, the resulting film is amorphous and insulating. A high-temperature post-deposition anneal at 800.degree.-900.degree. C. in oxygen is required to convert the amorphous precursor film into a super-conducting film with the correct crystal structure. This two-step process is referred to as a "post-anneal" process. However, when a thin-film such as YBa.sub.2 Cu.sub.3 O.sub.7-x is deposited onto a substrate heated to about 550.degree.-750.degree. C., the resulting thin-film is crystalline and superconducting as deposited. This process is referred to as an "in-situ" process.
Films produced by an in-situ process have less surface roughness and generally superior properties when compared to films from a post anneal process. In-situ deposition has the added advantage of lower processing temperature requirements which makes in-situ deposition compatible with a wider variety of substrate materials.
In-situ processes require heating the substrate uniformly and reproducibly to the required deposition temperature. These methods involve direct thermal and mechanical anchoring of the substrates to a heated block as discussed in R. W. Simon, SPIE Proceedings Vol. 1187, p. 2, edited by T. Venkatesan, published by SPIE Bellingham, WA (1989). The heated block is made of a material that is compatible with an oxygen atmosphere and has good thermal conductivity such as nickel. Good thermal contact between the substrate and the block is usually provided by a compound with high thermal conductivity such as silver paste.
The methods of the prior art have been useful to provide thin films of oxides of relatively involatile oxides (as defined below). The prior art methods of forming in-situ films by vapor deposition onto substrates heated to elevated temperatures, however, tend to cause loss of volatile oxides from the film during the deposition. The resulting film tends to lack sufficient volatile oxide to produce the desired stoichiometry and crystal structure. Although substrates heated to lower temperatures may be employed during deposition, the films deposited on those substrates are amorphous and therefore must be subjected to an additional annealing step subsequent to deposition.
A need therefore exists for a method of producing in-situ thin films of materials that comprise volatile oxides but which avoids the disadvantages of the prior art methods.